In the field of television broadcasting, DTV (digital TV) system is a new generation of recent years, which is an all-digital system from REC to transmission, then to reception. DVB-T is one of the four kinds of DTV broadcasting standards in the world, and DVB-H is a standard for handheld application based on DVB-T standard. Both DVB-T and DVB-H are based on Orthogonal Frequency Division Multiplexing (OFDM). It has gained much attention and been put in use in about 100 countries. The recent researches on the relevant technologies have become very important for the great market value of DVB-T/H.
In practical communication system, modulator and demodulator usually work either at baseband or at an intermediate frequency (IF). As we have to transmit signal at some allocated radio frequency (RF), it follows that we have to up-convert the modulated signal to RF channel in the transmitter, and down-convert RF signal to IF or baseband in the receiver. To do this the practical local oscillators will be used, which bring phase noise (PHN) and interfere with the signal. Since the receivers are usually manufactured at low cost, PHN is considered more in the receiver end.
The phase noise is not a big problem for the conventional analog TV systems, but the significance of the problem increases very strongly with the introduction of OFDM. The main difference between the OFDM and other digital modulation types is that the OFDM signal consists of multiple low rate sub-carriers that are orthogonal with each other, therefore the OFDM systems are very sensitive to phase noise. The low symbol rate makes the synchronization even more difficult when fast phase disturbances occur, and the phase noise degrades orthogonality of the sub-carriers.
When PHN has been added to the OFDM signal at a DVB-T receiver, the received OFDM signal will suffer two effects: Common Phase Error (CPE), which is a rotation of the signal constellation, and also known as average phase noise offset affecting all the sub-channels equally, and Inter Carrier Interference (ICI), similar to additive Gaussian noise, which is caused by the loss of orthogonality of the sub-carriers and will interfere with demodulation of OFDM symbols. Therefore, PHN removal is very important for a practical OFDM system.
The Common Phase Error (CPE) can be removed in the receiver and the Inter Carrier Interference (ICI) effect only can be reduced. Because the information of pilots inserted in the OFDM symbol are known to the receiver, the CPE removal methods are based on pilots. FIG. 1 is a block diagram of partial configuration of a conventional OFDM signal receiver 100, which includes a demodulation module 110, a Fast Fourier Transform (FFT) module 120, a synchronization and timing (S&T) module 130, a Common Phase Error (CPE) removal module 140, a channel estimation (CE) module 150, and a channel decoding module 160. The demodulation module 110 demodulates a digital OFDM signal received to generate a complex signal including an in-phase signal and a quadrature-phase signal. The digital signal compensated by the synchronization and timing module 130 passes through the FFT module 120. Then the CPE removal module 140 estimates and corrects a Common Phase Error (CPE) contained in all sub-carriers of the OFDM digital signal. Afterwards the corrected digital signal is equalized in the channel estimation module 150, and then transmitted to the channel decoding module 160 for the decoding process.
There are mainly three types of conventional solutions for the CPE estimation, wherein the first one is a cross correlation method, the second one employs an auto-correlation method, and the third type of the conventional CPE estimation employs a two-steps method, as illustrated respectively through FIGS. 2A to 2C. The detailed algorithm of conventional CPE estimation method will be discussed in the following paragraph.
Assumed the useful length of an OFDM symbol is Tu, the number of sub-carriers is N, the guard interval (GI) is L, then the modulated symbol in time domain is:
                                                        x              i                        ⁡                          (              n              )                                =                                                    ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                                    X                    i                                    ⁡                                      (                    k                    )                                                  ⁢                                  ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                    kn                                                  ⁢                                                                  ⁢                n                                      =                          -              L                                      ,                  …          ⁢                                          ⁢          0                ,        1        ,                              …            ⁢                                                  ⁢            N                    -          1                                    (        1        )            
The signal received in the receiver is:yi(n)=xi(n)ejφi(n)hi(n)+μi(n)  (2)
Where φi(n) is the CPE, hi(n) is the channel response, μi(n) is the AWGN.
After FFT, the signal in frequency domain is:Yi(k)=Xi(k)ejφi(k)·Hi(k)+μi(k)  (3)
Then assumed the local pilot sub-carriers in receiver is Pk(k=1, . . . , K), K is the number of pilots used in a symbol. For the discussion convenience, we assume |Hi(k)|=1, which can simplify the analysis.
As shown in FIG. 2A, when the cross-correlation method is engaged in the first type of conventional CPE estimation, we can obtain:
                                                                        φ                ^                            =                            ⁢                              arg                ⁡                                  (                                                            P                      k                                        ⁢                                                                  ⅇ                                                                              jφ                            i                                                    ⁡                                                      (                            k                            )                                                                                              ·                                                                        H                          i                                                ⁡                                                  (                          k                          )                                                                    ·                                              P                        k                        *                                                                              )                                                                                                        =                            ⁢                                                                    φ                    i                                    ⁡                                      (                    k                    )                                                  +                                  arg                  ⁡                                      (                                                                  H                        i                                            ⁡                                              (                        k                        )                                                              )                                                                                                                          =                            ⁢                                                                    φ                    i                                    ⁡                                      (                    k                    )                                                  +                                                      φ                    H                                    ⁡                                      (                    k                    )                                                                                                          (        4        )            
This method based on cross-correlation is the simplest. But it can not obtain good performance as it does not consider the channel effect. The CPE estimated results have a term of channel response. Under the multi-path channel or Doppler condition, the CPE estimation results will have great estimated errors since the channel response is not linearity.
In FIG. 2B, when the auto-correlation method is engaged, we can obtain:
                                                                        Δ                ⁢                                  φ                  ^                                            =                            ⁢                              arg                ⁡                                  (                                                            P                      k                                        ⁢                                                                  ⅇ                                                                              jφ                            i                                                    ⁡                                                      (                            k                            )                                                                                              ·                                                                        H                                                      i                            +                            1                                                                          ⁡                                                  (                          k                          )                                                                    ·                                                                        (                                                                                    P                              k                                                        ⁢                                                                                          ⅇ                                                                                                      jφ                                                                          i                                      -                                      1                                                                                                        ⁡                                                                      (                                    k                                    )                                                                                                                              ·                                                                                                H                                  i                                                                ⁡                                                                  (                                  k                                  )                                                                                                                                              )                                                *                                                                              )                                                                                                        =                            ⁢                              arg                ⁡                                  (                                                            P                      k                                        ⁢                                                                  ⅇ                                                                              jφ                            i                                                    ⁡                                                      (                            k                            )                                                                                              ·                                                                        H                                                      i                            +                            1                                                                          ⁡                                                  (                          k                          )                                                                    ·                                                                        (                                                                                    P                              k                                                        ⁢                                                                                          ⅇ                                                                                                      jφ                                                                          i                                      -                                      1                                                                                                        ⁡                                                                      (                                    k                                    )                                                                                                                              ·                                                                                                H                                  i                                                                ⁡                                                                  (                                  k                                  )                                                                                                                                              )                                                *                                                                              )                                                                                                        =                            ⁢                                                                    φ                    i                                    ⁡                                      (                    k                    )                                                  -                                                      φ                                          i                      -                      1                                                        ⁡                                      (                    k                    )                                                                                                          (        5        )                                          φ          ^                =                                            φ              i                        ⁡                          (              k              )                                =                                                    φ                                  i                  -                  1                                            ⁡                              (                k                )                                      +            Δφ                                              (        6        )            
In this auto-correlation method, the difference of PHN of the neighbor symbol is estimated, so the PHN result can be obtained by sum of estimated result.
After CPE removal, we can obtain:
                                                                                                              Y                    ^                                    i                                ⁡                                  (                  k                  )                                            =                            ⁢                                                                    Y                    i                                    ⁡                                      (                    k                    )                                                  ·                                  ⅇ                                                            -                      j                                        ⁢                                          φ                      ^                                                                                                                                              =                            ⁢                                                                                          X                      i                                        ⁡                                          (                      k                      )                                                        ⁢                                                            ⅇ                                                                        jφ                          i                                                ⁡                                                  (                          k                          )                                                                                      ·                                                                  H                        i                                            ⁡                                              (                        k                        )                                                              ·                                          ⅇ                                                                        -                          j                                                ⁢                                                  φ                          ^                                                                                                                    +                                                      μ                    i                                    ⁡                                      (                    k                    )                                                                                                                          =                            ⁢                                                                                          X                      i                                        ⁡                                          (                      k                      )                                                        ·                                                            H                      i                                        ⁡                                          (                      k                      )                                                                      +                                                      μ                    i                                    ⁡                                      (                    k                    )                                                                                                          (        7        )            
After CE (channel estimation), we can obtain:
                                                                                                              X                    ^                                    i                                ⁡                                  (                  k                  )                                            =                            ⁢                                                                                          Y                      ^                                        i                                    ⁡                                      (                    k                    )                                                  ·                                                                            H                      ^                                        i                    *                                    ⁡                                      (                    k                    )                                                                                                                          =                            ⁢                                                (                                                                                                              X                          i                                                ⁡                                                  (                          k                          )                                                                    ·                                                                        H                          i                                                ⁡                                                  (                          k                          )                                                                                      +                                                                  μ                        i                                            ⁡                                              (                        k                        )                                                                              )                                ·                                                                            H                      ^                                        i                    *                                    ⁡                                      (                    k                    )                                                                                                                          =                            ⁢                                                                                          X                      i                                        ⁡                                          (                      k                      )                                                        ·                                      ⅇ                                          j                      ⁡                                              (                                                                                                            φ                              H                                                        ⁡                                                          (                              k                              )                                                                                -                                                                                                                    φ                                ^                                                            H                                                        ⁡                                                          (                              k                              )                                                                                                      )                                                                                            +                                                      μ                    i                                    ⁡                                      (                    k                    )                                                                                                          (        8        )            
The auto-correlation method can obtain better performance than the cross-correlation method. Since the channel effect can be almost eliminated by auto-correlation operation. But it has more implementation complexity than the cross-correlation method.
Because information obtained by CE has some difference with the real channel response, there will be some residual phase errors after CE. The methods before CE are called pre-CE methods, which have no benefit for phase error compensation after CE.
As illustrated in Fig.2C, the conventional two steps method is shown, which can resolve above mentioned problem. Besides of CPE removal before CE, the fine CPE removal after CE is engaged. The part before CE is the same as the auto-correlation method, and the fine CPE estimation is like to the cross-correlation method. The fine removal process after CE will be not affected by channel conditions and it can eliminate the residual phase error φH(k)−{circumflex over (φ)}H(k) in equation (8). So the two steps method is more accurate than the pre-CE methods. However, the main shortage of the method is that it has much higher complexity as it can be seen the twice CPE removal.
Therefore, it is desirable to develop an improved method for CPE removal of the OFDM signal reception, which overcomes drawbacks of the prior arts.